Relay valve

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A relay valve is an air-operated valve. It is most-commonly used in air brake systems to control the brakes at the rear of a trailer in a tractor-trailer truck.

It would take too long for sufficient air to travel from the brake pedal valve to the rear of the trailer to operate the brakes in conjunction with the other brakes. Therefore an air supply is stored at the rear of the trailer whenever the emergency brakes are released. When the driver presses the brake pedal a small amount of air opens the relay valve which directs air from the local source directly to the rear brakes.

An emergency relay valve works on the same principle as a standard relay valve. The emergency function in it acts if a major drop in air pressure occurs from the coupling. In this scenario the air passes from the park system to the service line thus allowing the vehicle to stop safely, instead of all the air being dumped and the trailer park springs applying.

A relay valve is analogous to a pneumatic transistor.

Relay Valve The foot valve is usually located closer to the front wheels than to the rear wheels. The longer the distance from the foot valve to the rear chambers, the more time it will take before the rear brakes apply. This is known as brake lag. To correct this condition on a long wheel base vehicle, a relay valve (13) is installed near the rear brake chambers. A larger diameter pipe is connected between the main reservoir and the relay valve. The air line from the foot valve to the relay valve now becomes a “control line.” (The air in the control line “dead ends” at the relay valve.) When the foot valve is depressed, the air pressure in the control line acts on the top section of the relay valve, relaying reservoir air directly to the rear brake chambers through the larger diameter pipe. The pressure of the reservoir air delivered in this way will be the same as the control pressure delivered by the foot valve.

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